1. Field of the Invention
The present invention relates to a method and device for determining and indicating the amount of residual charge remaining in a battery, and more particularly to a method and device for determining the residual capacity of a battery when a nickel-cadmium battery or the like is connected to a battery charger equipped with a detector for detecting a fully charged condition and is charged thereby.
2. Description of the Related Art
When a secondary cell such as a nickel-cadmium battery or the like, the output voltage of which is substantially constant, is charged and reaches a fully charged state, a trickle charging current from the battery charger often continues indefinitely, so that it is desirable to determine whether or not a charging operation has been completed. For this reason, it is necessary to connect the secondary cell or battery not only to a trickle charger that provides a so-called microcurrent from the beginning of a charge cycle in the form of a charging current, but also to a charger that has a function of detecting a fully charged state so that a change in the charging current may be made immediately before or after a constant charge has been completed.
In general, in the case of the prior art where the residual capacity of a battery is indicated, there is a well known method and apparatus for measuring and indicating a charging current and discharging current, hereinafter referred to as a charging amount and discharging amount, respectively.
The above-noted charging amount is indicated by a value I.sub.c .times.T.sub.c by which a charging current I.sub.c is multiplied by a time Tc and, for example, if a charging current Ic of 50 mA is maintained for ten hours, the charging amount would be I.sub.c .times.T.sub.c =500 mAh. When a value of 500 mAh is reached, if the charger detects a full charge, the value of 500 mAh is considered to be the full charging amount.
The discharging amount is designated by the product of I.sub.D .times.T.sub.D by which a discharging current I.sub.D is multiplied by a discharging time T.sub.D. If I.sub.D is 50 mA and the discharging time is 10 hours, the full discharging amount is 500 mAh, which is designated a dischargeable amount. For a time until a discharge-terminating voltage is reached so that the voltage between both terminals of a battery becomes 1V per cell, 10 mA per hour is discharged and the discharged amount after ten hours is, for example, I.sub.D .times.T.sub.D =100 mA, the residual capacity of a battery at this time, or the remaining discharge amount, is indicated as 500-100=400 mAh.
In the prior art method and device for indicating the residual capacity of a battery, it is assumed that all of the charged amount can be discharged. This causes a problem that the charging amount and the discharging amount are not linearly related on a 1 to 1 ratio based on the charging efficiency or the surrounding environment and an error in the indication of the remaining capacity may occur in the last stage of the discharge processes.